It is well known in the art of diamond synthesis that atomic hydrogen is an important reactive species in diamond growth at low pressures (1-760 tort) by chemical vapor deposition (K. E. Spear, J. Am. Ceram. Soc. 72, 171, 1989). It is believed that atomic hydrogen, created by dissociating molecular hydrogen, establishes active sites on an otherwise hydrogenated (and hence stabilized) diamond surface, through a dynamic balance of abstraction of surface hydrogen atoms and chemisorption. These active sites are then free to react with elemental carbon or hydrocarbon precursors. The surface complex subsequently reacts with atomic hydrogen to ultimately incorporate the transported carbon into the growing diamond lattice. Thus, diamond growth rates are favored by the production or availability of atomic hydrogen and the rate of transport of atomic hydrogen to a growing diamond surface.
It is also known that OH radicals (which, along with H, are the products of water decomposition in plasma environments, the intermediate products of the combustion of most fuels, and a major species in the high temperature reaction of molecular oxygen and dissociated hydrogen) are also important reactive species in diamond growth at low pressures by chemical vapor deposition (M. A. Cappelli and P. H. Paul, "An Investigation of Diamond Synthesis in a Premixed Oxy-Acetylene Flame", J. Appl. Phys. 67, 2596, 1990). OH radicals can play a similar role to that of atomic hydrogen in "stabilizing" the surface of diamond (preventing surface reconstruction) and is believed to be more efficient at "etching" graphite (D. E. Rosner, "High Temperature Gas-Solid Reactions," Ann. Rev. Mater. Sci. 573, 1972).
Higher growth rates are favored at higher pressures. Diamonds have been synthesized by chemically transporting carbon atoms in liquid metal solutions at high pressure (greater than 5000 atmospheres) and high temperatures (typically 1500.degree. K.) through a temperature gradient to condense onto diamond seed crystals. The carbon atoms are supplied from forms of carbon such as graphite which are soluble in the liquid metal. (R. C. Devries, "Diamond Synthesis Under Metastable Conditions," Ann. Rev. Mater. Sci., 161; 1987).
Supercritical water is defined as water maintained above its critical pressure (218 atm) and temperature (374.degree. C.). Supercritical water is known to be an effective solvent for large hydrocarbons and alcohols O. F. Connolly, "Solubility of Hydrocarbons in Water Near the Critical Solution Temperatures," J. Chem. Engineering Data 11, 13, 1966). Such an environment is also known to be a suitable reactive environment for the oxidation of organics (T. B. Thomason and M. Modell, "Supercritical Water Destruction of Aqueous Wastes," Hazardous Waste 1, 453, 1984; M. Modell, "Processing Methods for the Oxidation of Organics in Supercritical Water", U.S. Pat. No. 4,543,190, 1985) and the breakdown of carbonaceous materials such as a variety of coals. (I. R. Kershaw and L. J. Bagnell, "Extraction of Australian Coals with Supercritical Aqueous Solvents," in Supercritical Fluids (T G. Squires and M. E. Paulaitis, Eds., Amer. Chem. Soc., Washington, DC), 1987, p. 266; D. S. Ross, G. P. Hum, T-C. Miin, T. K. Green, and R. Mansuni, "Isotope Effects in Supercritical Water: Kinetic Studies of Coal Liquefaction," in Supercritical Fluids, (TG. Squires and M. E. Paulaitis, Eds., Amer. Chem. Soc., Washington, DC, 1987, p. 242).